1. Field of the Invention
The present invention relates generally to varifocal lens assemblies and more particularly to a varifocal lens assembly having a macro-photography capability.
2. Description of the Prior Art
As generally known by optical designers, a varifocal optical system provides a lens system in which the focal length or image magnification can be continuously varied, while a relatively sharp image is automatically maintained in a fixed plane within tolerable limits. It has been known that a continuously variable focal length is achieved by displacing the optical components of a system relative to each other. This effect however, is accompanied by a displacement of the image plane of the system. Thus, the optical designer of a varifocal system, has been continuously faced with the problem of compensation for image shifts, in addition to the problem of correction of aberrations for each position of the movable lens components.
In early designs, the image shift compensation was achieved by mechanical means. Generally, these mechanical designs required imparting two or more coordinated movements to the lens elements of the system. The first movement would effect the change in overall focal length while the second movement of the lens would compensate for the image shift. In addition, a third movement would be added for the purpose of changing the size of the aperture stop to keep the light output of the lens constant across the variable focal lengths. Examples of these equivalent varifocal lens systems, can be seen in the Warmisham et al, U.S. Pat. No. 1,947,669; Naumann U.S. Pat. No. 1,988,390 and Mellar el al, U.S. Pat. No. 2,159,394. These movements in the mechanical image shift compensation lens systems were nonlinearly correlated to each other, thus, necessitating comparatively complicated cam controls and precise tolerances.
An optical image shift compensation was first suggested at the turn of the century, for example, Allen U.S. Pat. No. 696,788. The theory behind an optical image shift compensation has been set forth in GENERAL THEORY OF OPTICALLY COMPENSATING VARIFOCAL SYSTEMS by Bergstein, Journal of the Optical Society of America, Volume 48, No. 3, P. 154, March 1958, wherein a general analytical theory of the Gaussian optics of optically compensated varifocal systems is fully developed. The contents of this article is hereby incorporated by reference for purposes of supplementing the disclosure of the present invention.
Generally, in an optically compensated varifocal system, alternately stationary and relatively fixed movable lens components can be arranged to produce an optical system which will have an overall focal length variable continuously between any two predetermined values upon a displacement of the movable components from their reference position. While the movable interconnected system is being displaced from one extreme position to the other to change the overall focal length the final image of the system will pass through the same position at least once. The deviation of the image plane from a predetermined reference plane, e.g., film plane, will be zero at least once and possibly more depending upon the number and arrangement of movable lens components.
The optical designer of a zoom lens will attempt to maximize those points of zero divergence while minimizing the deviations between the zero points. As is shown, the depth of focus especially of objects a considerable distance from the lens system provides the designer with a degree of tolerance in positioning the film plane. The relative distances of the objects lying in a plane conjugate to that of the photographic plate or film can vary within a certain range and still provide acceptable photographic objects. The range of object distances or depth of field will vary relative to the distance of the object from the lens system. Optical designers of zoom lens assemblies are fully cognizant of the tolerances permissible while still maintaining a commercially acceptable photograph.
Reference is made to the article, "TWO-COMPONENT OPTICALLY COMPENSATED VARIFOCAL SYSTEM" by Bergstein et al, Journal of the Optical Society of America, Volume 52, No. 4, P. 353, April 1962, wherein the design of a two-component optically compensated varifocal lens system is disclosed. Basically, the design parameters are based on a math model wherein the image plane deviations are measured relative to the movement of one element of the lens system. In optimizing the lens design, consideration is given to the particular use of the lens system so that the image deviations can be reduced to zero at the appropriate positions. In a two-lens component system the image plane will pass through the same predetermined position and space at only two positions. Thus, the image plane deviations can be reduced to zero at two separate points, that is, the image will be formed correctly in conjugate with the photographic plane in two positions. The defect of focus at intermediate points will generally follow the parabolic law. With a relatively simple two-component zoom system, the parameters of the system is generally chosen to minimize the image deviation within acceptable minimum tolerances throughout the entire desired focal range.
In the above article, entitled "TWO-COMPONENT OPTICALLY COMPENSATED VARIFOCAL SYSTEM", one such lens system which has a maximum over-all focal length when the movable components are in a front position is shown. This lens system comprises a first lens group of negative refractive power and a second lens group of positive refractive power movable along the optical axis relative to the first lens. The focal length of a system of this type, is continuously varied in the course of the movement of the second lens group from one extreme to another extreme of its movable mounting.
During the variance of the focal length of the system the image plane deviation follows a parabolic law wherein the image plane of the system is shifted from an initial position towards the object side of the design focal plane then is shifted back again to the same initial position at the other extreme of the focal length range. In the design of a normal two-component zoom lens, the absolute image plane deviation, would be designed to be maintained within permissible tolerances whereby the depth of field would be of such a size for the particular focal length that all objects within this range would be simultaneously "in focus" on the photographic film or plane. Reference is made to both of the above mentioned articles in the Journal of the Optical Society of America for the theory in designing a lens system taking into consideration the image plane deviation characteristics of varifocal lens systems.
Briefly, the reflective power, f.sub.1, of a first lens group and the reflective power, f.sub.2, of the second lens group are given as follows: ##EQU1## wherein R is the ratio of the maximum to minimum overall focal length of the combined first and second lens groups and S.sub.12 is a distance between the first lens group, which for purposes of analysis is represented as a thin lens and a second lens group, which is also represented by a thin lens, when the first and second thin lens groups are in a position having the maximum focal length.
In accordance with the design objectives, of using a varifocal lens system as a zoom lens, the focal plane of the back focal length of the lens system is positioned relative to the depth of field to be midway between the displacements of the image plane across the range of the focal length of the zoom lens, thus, by picking the appropriate setting of the back focal length, the zoom lens assembly will be in focus with an object set at an infinite distance across the entire range of focal lengths, the image plane being that of the object set at infinity.
In accordance with the conventional characteristics of a zoom lens, the setting of the back focal length with appropriate consideration taken for the null points of image plane deviation permits the extreme positions of image plane deviation to be within the depth of focus or depth of field of the lens system. Accordingly, there is no loss of focus, with regards to acceptable photographic tolerances, throughout the variation of a focal length when the above mentioned extremes of image plane deviations are both within the depth of focus of the system. As understood by the lens designer, the degree of image plane deviation that is commercially acceptable depends upon the depth of field required, a large depth of field will provide a relatively large depth of focus, however, in the field of macro-photography or close-up photography, a relatively narrow depth of field will require a corresponding relatively narrow depth of focus.
It has been known to use zoom lens assemblies for macro-photography in various ways. For example, a separate close-up lens attachment or an extension tube attachment has been combined with a zoom lens assembly to provide macro-photography. This compounding of lens accessories on the basic camera body is both burdensome, expensive and unwieldy during normal use by a photographer. It has also been suggested that a zoom lens be designed, wherein the lens components are given a special movement during macro-photography. This special movement being different from the movement utilized during the zooming photography across the range of focal lengths. As mentioned earlier, with regard to mechanical compensation of image shift, the use of various lens arrangements wherein coordinate movement of the lenses are required, further adds to the complication of the basic lens system, increases the size of the construction of the lens system and requires additional cost. The ability to provide a variable focal length with a capability for macro-photography in a relatively simple and uncomplicated lens system has not been provided by the prior art.